The present disclosure relates to a detection apparatus for detecting existence of a conductor such as a metal between a power transmitting apparatus and a power receiving apparatus. In addition, the present disclosure also relates to the power receiving apparatus, a non-contact power transmission system and a detection method.
In recent years, a non-contact power transmission system for supplying electric power by adoption of a wireless technique is developed aggressively. Methods for supplying electric power by adoption of a wireless technique include two methods to be described as follows.
One of the two methods is an electromagnetic induction method which is already known widely. In the electromagnetic induction method, the degree of junction between the power transmitting side and the power receiving side is very high so that electric power can be supplied from the power transmitting side to the power receiving side with a high degree of efficiency. Since the coefficient of junction between the power transmitting side and the power receiving side needs to be held at a high value, however, the efficiency of power transmission between coils on the power transmitting side and the power receiving side deteriorates considerably if the distance between the power transmitting side and the power receiving side increases or if the power transmitting side is shifted from a position exposed to the power receiving side. In the following description, the efficiency of power transmission between coils on the power transmitting side and the power receiving side is also referred to as an inter-coil efficiency.
The other method is a technique referred to as a magnetic resonance method. The magnetic resonance method is characterized in that, by utilizing a resonance phenomenon deliberately, the magnetic flux shared by the power supplier and the power receiver is small. In the magnetic resonance method, even for a small coefficient of junction, the inter-coil efficiency does not deteriorate provided that the Q factor (quality factor) is high. The Q factor is an indicator representing a relation between energy holding and energy losing in circuits including the coils on the power transmitting side and the power receiving side. That is to say, the Q factor is an indicator representing the strength of resonance of a resonant circuit. In other words, the magnetic resonance method offers a merit that the axis of the coil on the power receiving side does not have to be adjusted to the axis of the coil on the power transmitting side. Other merits include a high degree of freedom in selecting the positions of the power transmitting side and the power receiving side as well as a high degree of freedom in setting the distance between the power transmitting side and the power receiving side.
One of important elements in the non-contact power transmission system is a countermeasure to be taken against heat dissipated by a metallic foreign substance. When electric power is supplied from a power transmitting side to a power receiving side by adoption of a non-contact technique not limited to the electromagnetic induction method or the magnetic resonance method, a metal may exist between the power transmitting side and the power receiving side. In this case, an eddy current may flow in the metal so that it is feared that the metal dissipates heat. As a countermeasure to be taken against the heat dissipated by the metal, there have been proposed a number of techniques for detecting such a metallic foreign substance. For example, techniques making use of a light sensor or a temperature sensor are known. However, a method for detecting a metal by making use of a sensor is expensive if the power supplying range is wide as is the case with the magnetic resonance method. In addition, if a temperature sensor is used for example, a result output by the temperature sensor is dependent on the thermal conductivity of the surroundings of the sensor. Thus, design restrictions are imposed on equipment on the power transmitting side and the power receiving side.
In order to solve the problems described above, there has been proposed a technique for determining whether or not a metallic foreign substance exists between the power transmitting side and the power receiving side through examination of parameter changes caused by existence of the metallic foreign substance. The changes of parameters typically include changes in current and changes in voltage. By adoption of such a technique, it is no longer necessary to impose design restrictions on equipment on the power transmitting side and the power receiving side. In addition, the cost can be reduced. As described in Japanese Patent Laid-open No. 2008-206231 (hereinafter referred to as Patent Document 1) for example, there has been proposed a technique for detecting a metallic foreign substance through examination of the degree of modulation in transmission between the power transmitting side and the power receiving side. That is to say, a metallic foreign substance is detected by examining information on amplitude changes and phase changes. In addition, as described in Japanese Patent Laid-open No. 2001-275280 (hereinafter referred to as Patent Document 2), there has been proposed a technique for detecting a metallic foreign substance through examination of an eddy-current loss. This technique is also referred to as a foreign-substance detection method based on a DC-DC efficiency.